The present invention relates to surgically implantable prosthetic bone joints and more particularly to prosthetic ball-and-socket bone joint devices having a replaceable bearing insert present in the socket on which the ball articulates.
Disease or trauma involving the bones of a joint could necessitate that the natural joint be partially or wholly replaced by implanting a joint prosthesis. Joint prostheses have been developed to replace natural ball-and-socket joints, such as those found in the shoulder and the hip.
For example, for restoration of a hip joint, a prosthesis typically consists of a femoral component which includes a stem adapted to be introduced into a surgically prepared intramedullary canal of a femur, a neck attached to the proximal end of the stem portion, and a head portion attached to the neck. The head portion could be designed to articulate with the concave portion of the acetabulum or with a concave acetabulum component secured to the acetabulum, either of which constitute the socket.
It is known in the art that articulation can be improved by incorporating a component of low friction material to serve as one of the articulating surfaces. Providing for substantially frictionless articulation allows for easier movement of the joint, decreased torsional stresses and their effects on the joint components, extended implant life, and less pain for the patient.
Several designs developed make use of a low-friction material, e.g. ultra-high molecular weight polyethylene (UHMWPE), to comprise one of the articulating surfaces. Some of these designs include a ball component on the neck of the femoral component, a low-friction bearing surface surrounding a portion of the ball on which the ball articulates, and an outer shell permanently retaining the bearing surface. These prostheses can be assembled merely by popping the ball component into the bearing-lined outer shell. The bearing inner surface is generally spherical to mate with the ball component and, if the bearing surface and/or the outer shell surrounds more than half of the ball, the ball is retained inside the outer shell. These designs risk separation of the ball component from the outer shell if insertion and removal of the ball component is relatively easy.
If the device is designed so that the ball component is not easily separated, then a relatively large amount of force is required to insert the ball component, making it difficult for the surgeon to insert and more painful for the patient. Alternatively, the procedure of assembly may include heating the plastic insert to allow it to expand for insertion of the ball component. This adds an additional and time-consuming step during surgery. With these types of prostheses subsequent operations to remove the ball component to revise the joint are difficult, possibly requiring removal of the femoral stem if separation of the ball from the bearing component is too difficult.
In an attempt to lower the risk of separating the ball component from the shell yet ensure that revision of the hip joint with different prosthetic components is possible without having to remove the stem from the femur, several designs have been developed that make use of a bearing component with a means for locking and unlocking the ball component.
Designs which have non-replaceable inserts include those disclosed in U.S. Pat. Nos. 4,408,360 and 4,241,463. Non-replaceable inserts have the disadvantage of requiring replacement of both the outer shell and the insert after wear of the insert or for revision to a different size outer shell.
Prostheses with replaceable inserts of multiple components have been disclosed in U.S. Pat. No. 4,380,090 to Ramos, U.S. Pat. No. 3,848,272 to Noiles and U.S. Pat. No. 3,978,528 to Crep. Replaceable inserts are those that allow for assembly and disassembly of the entire prosthesis with little or no damage to the insert. Ramos discloses a three-component insert which includes a bearing insert, an annular bearing, and a retaining ring. Noiles shows an embodiment that includes two inserts and a flexible ring member. Another embodiment shown by Noiles is a one- or two-piece insert that is threaded for screwing into the acetabulum prosthesis. Crep shows a split lining (two-piece) insert and a locking ring. The multiple-piece inserts have the disadvantages of having multiple pieces to assemble and account for and having the potential that several pieces would be loose inside the body if dislocation occurred. The threaded inserts of Noiles are heated to cause expansion in order for the hip ball to be popped in and further risk gradual unthreading or loosening of the insert during use.
Two-piece bearing inserts have been developed by Orthopedic Equipment Company (OEC), DePuy, and Biomet Inc. These inserts not only require the manufacture and assembly of the two pieces, but also risk having two insert pieces loose in the body in the event of dislocation. The loose pieces could cause damage to the body internally and would preclude external reduction.
Orthopedic Equipment Company sells OEC Universal Self-Aligning Acetabular Components, as shown in Form No. 01-50-1422, which includes two pieces: a UHMWPE inner bearing and a split titanium retaining ring. The inner bearing is slotted twice from the bottom of the bearing to the apex leaving a hinged portion at the apex, and the retaining ring is fitted into a groove on the outside of the bearing. When the prosthesis is assembled, the retaining ring is also locked into a groove in the inside of a metal outer bearing. During disassembly, removal of the inner bearing from the outer bearing requires a specially-made releasing ring.
Biomet Incorporated sells components under the name BI-POLAR Articulating Components, and in Brochure No. Y-BMT-011/010184 (1984), "BI-POLAR Articulating Components - Surgical Technique", the components are described as a metal outer shell with an UHMWPE surface fitted inside the shell which is secured to the femoral implant head by means of a slotted UHMWPE locking ring. The outside of the ring has contours and locking ridges to match the inner shell and fits below the centerline of the femoral head. Assembly of the prosthesis is by first placing the locking ring over the femoral head, sliding on the shell, then securing the shell to the locking ring. For disassembly, the locking ring can be released by inserting instrument tips in the holes in the locking ring and compressing the slot closed.
DePuy, a division of Boehringer Mannheim Corporation, sells a hip prosthesis under the name, Self-Centering.RTM. Universal Hip shown in Brochure 382 0601-52 (Rev. 1) (1982), which includes an outer metal cup with an inner annular groove, a polyethylene insert, and a locking ring. The polyethylene insert is generally hemi-spherical on its outside and inside surfaces with a deep annular groove on the outside near the bottom or open end of the insert. The locking ring is slotted vertically and has an annular rib on the outside surface and an annular lip on the inside surface at the top of the ring designed to fit into the groove of the insert. Assembly is accomplished by connecting the insert and the locking ring together and snapping the assembly onto the femoral component head and sliding the metal cup over the assembly. The annular rib on the locking ring locks into the metal cup's annular groove. For disassembly, release forceps are inserted into holes in the bottom of the locking ring to close the slot, thus releasing the rib from the groove. To make this insert assembly, narrow design tolerances are required for a secure fit between the locking ring and the insert.
The two-piece bearing inserts described above not only require the manufacture and assembly of two insert pieces, but also risk having two insert pieces loose in the body in the event of dislocation. The loose separated pieces could cause further damage to the body internally and would preclude external reduction.
3M sells a femoral head prosthesis by the name of Bateman UPF.RTM. II that has a one-piece polyethylene bearing insert. Brochure No. SD-IUTS(421)NPI (1982) shows that the interlock between the outer head rim and the insert segments is accomplished by a lip on the bottom inside of the outer head rim seating into a notch on the outside bottom of the bearing insert. On the outside surface of the side of the insert there is an annulus forming a recess. The insert is slotted from the bottom to the annulus in six places, thereby forming six solid segments. The insert encompasses more than one-half of the metal head to retain it. Removal of the outer head is accomplished by (1) inserting tool or bone screws into two holes in two opposing segments of the bearing insert, (2) pressing the holes together slightly to disengage the interlock, and (3) slipping the outer head off. The product brochure indicates that this prosthesis design is covered in U.S. Pat. No. 3,863,273. A prosthesis with an easier method of disassembly is still desired.